1. Field of the Invention
The present invention relates to a control system and a method of a lockup control for a vehicular automatic transmission and particularly to a hydraulic pressure control of the lockup clutch when a lockup is operative.
2. Prior Arts
In modern automatic transmissions, an electronic transmission control is becoming popular and a lockup control in an automatic transmission is no exception. In a case of the lock-up control, for example, when an ATF (automatic transmission fluid) temperature is above a predetermined value and when the gear is at the positions ranging from 2nd to 4th excepting 1st, the lockup clutch is electronically controlled for saving fuel at a normal mode and for increasing power at a power mode. Further, in controlling the lockup clutch controls for attenuating a shock at the engagement of the clutch and the like are performed by increasing hydraulic pressure gradually.
Generally, a torque capacity of the lockup clutch varies according to miscellaneous factors such as ATF temperature, hydraulic pressure, a friction coefficient of a clutch facing, engine torque or the like. Therefore, usually the lockup clutch is designed with a sufficient capacity so as not to lack in capacity under any conditions. For example, it is designed so as to have a large contact surface and a high hydraulic pressure for engaging the lockup.
However, there are so many cases where the hydraulic pressure as well as the torque capacity become excessively large, for example when the ATF temperature is low and the engine torque is small. As a result, this higher lockup pressure than necessary exerts an adverse effect on the durability and the life of the clutch facing of the lockup clutch.
With respect to the pressure control of the lockup clutch, for example, Japanese Unexamined Patent Application No. Toku-Kai-Sho 60-101357 discloses a technique that the clutch engagement force can be varied by switching the lockup pressure from a high mode to a low mode or vice-versa according to the slip ratio of the clutch, namely, the rotational speed ratio of an input to and an output from the lock-up clutch. By means of this pressure control, shocks, vibrations or the like can be reduced. Further, Japanese Unexamined Patent Application No. Toku-Kai-Sho 61-160662 discloses another technique for reducing shocks and the like in which the lockup pressure is increased step-by-step by use of an accumulator and a lockup control valve provided in the hydraulic circuit.
In those prior arts described above, the engagement force in the transfer stage from a released state to an engaged state is increased gradually or in a step-by-step manner for a smooth engagement. However, since the lockup pressure at the final stage is always controlled at a specified high value, there is still a fear that the life of the clutch is shortened.